Cost-efficient operation of a sea-going vessel, whether it be a tanker, a container ship, a floating production platform, or a cruiser, demands rigorous monitoring of the integrity of the hull. When referring to the integrity of the hull, it is important to note that this means much more than the ability of the structure to withstand the application of normal operating stresses. More particularly, in addition to the continuous relatively mild stresses applied to a vessel at rest in port, or at sea, vessels must also be able to withstand the sometimes every violent forces experienced by ships at sea during storm and other less than ideal conditions, whether they be waves, wind, or combinations of the same.
Key to the maintenance of the structural integrity of a ship is protection of the surface of the steel hull from the corrosive effects of sea water. As is well known, the salinity of sea water causes it to chemically attack the surface of a ship.
More particularly, and by way of example, the heavy salt content of ocean water will tend to eat away at the material of which metal vessel hulls are made. The result will initially be minor pitting, which tends to be localized on account of the configuration of the ship's hull, the materials of which the hull are made, joints between various components of the hull, and other factors. Eventually, such corrosion will compromise the overall integrity of the ship under normal operating conditions and, of course, could have catastrophic effects in the event of inclement weather or a collision with debris, another vessel, or on account of being run aground.
Even fiberglass vessels must be coated to be protected from other deteriorating elements in the sea, such as the build-up of organic materials, marine organisms such as barnacles, and the like.
Thus, the first line of defense is the protection of the hull of a ship with an appropriate coating. Such coatings are selected for their properties of long life in sea water, resistance to radiation, ability to discourage the growth of marine life on the surface of the hull of the ship, adhesion and general toughness, among others. Likewise, a crucial element of the survey of a ship, is the inspection of the coating, which is often the first element in the ship to indicate the onset of a problem in the overwhelming majority of situations.
For these reasons, the issuing and renewal of certificates of seaworthiness for ships is dependent upon a rigorous inspection of the hull of the ship in which certain characteristics are checked throughout the entire hull. In particular, the condition of the hull is checked for corrosion and/or irregularities. The nature and amplitude of any observable dents in the surface of the hull is also examined.
Generally, these inspections are carried out in dry dock. The experts of the official bodies authorized to carry out these inspections meticulously inspect, square meter by square meter, the hulls of ships which are submitted for inspection.
Putting a ship into dry dock is a very time consuming and expensive task to perform and results in greatly increasing the cost of maintaining a ship. This cost is particularly large in the case of a large container ship where each day out of service costs the owner tens of thousands of dollars, or in the case of floating production platforms, where the cost due to lost production can be orders of magnitude higher.
The disadvantages of this process are compounded in the case of high-tonnage ships, whether these be, for example, oil tankers, ore carriers or container ships, because the sizes of dry docks are becoming less and less able to accommodate the growing size of such ships, because the large commercial ports with a dry docks that are capable of accepting the ships are ill-equipped, and because of the actual cost of taking the ships out of service.
As a result of these problems, there has been proposed a process involving the installation of an apparatus incorporating a transceiver-carrying transport device or carriage which is capable of moving underwater over the surface to be inspected. In particular, the system described in the background of U.S. Pat. No. 4,308,600 contemplates a carriage which is capable of moving on the surface of the submerged structure to be inspected. The carriage carries a transducer, for example a television camera, which is connected in a closed circuit to a receiver located on the surface. The receiver is suitable for producing an image of the surface which the carriage is scanning.
In principle, such a system is intended to carry out, from a location on land, the multiple checks to which the hull of a ship must be subjected. Most importantly, it is intended to do this while the ship is afloat, thus avoiding the costs associated with submitting the ship to a costly dry dock procedure.
However, the inspection carried out in accordance with this system suffers because the position of the carriage or operating apparatus used on the surface over which it is moving cannot be determined very exactly at any instant. This information is of value, for example, in order to make it possible to make a comparison between deterioration measurements made by transceivers carried by the apparatus, during successive statutory surveys. Thus, dry dock surveys of hull condition remain the standard.
However, as alluded to above, present world facilities simply cannot accommodate the number of ships which need to be inspected. Accordingly, there are numerous vessels on the sea today which are far behind their inspection schedules. As is apparent, the same poses the possibility of many problems, some of potentially devastating magnitude.
More particularly, in addition to the cost attendant to the mechanically inefficient operation of a vessel, failure to perform maintenance on a preventative basis will, sometimes, have a dramatic impact on the cost of a repair. This is because of the fact that if a small problem is left unattended, continued corrosion will cause it to become a serious problem. Repair costs will be far in excess of the cost of a repair made when deterioration is just beginning to occur. More seriously, however, if the problem is left unattended, it may result in weakening the structure of the vessel to the point that the application of relatively routine forces will cause the structure to break down, not only again increasing the cost of repair, but also posing the potential for loss of the vessel and, far worse, environmental disaster.
As a consequence of the above problems, including the expense of conventional dry dock inspection procedures, attempts are being made to overcome the disadvantages of present submerged vessel inspection systems and processes. In this regard, the assignee of this application has proposed that the high resolution acoustic positioning systems developed by the assignee be employed in an underwater inspection.
Patent Cooperation Treaty International Application No. P. C. T./N 094/00060 discloses a diver delivered unit which may be operated directly by a diver or otherwise in a measurement area marked by a plurality of lights. The unit is delivered to the area of the hull of a ship afloat in a harbor by a diver. In accordance with the preferred embodiment described in this application, the unit is moved over the hull of the ship by a diver, although the possibility of moving the unit on the surface of the hull without a diver is vaguely alluded to. While this disclosure is somewhat general, it appears to suggest that magnets may be used to adhere the unit to the hull of a ship while the unit is being propelled along the surface of the ship by thrusters.
As alluded to above, the above Treaty filing suffers from the disadvantage of being dependent upon diver delivery. Moreover, the use of a plurality of lights to guide the system is an additional task with its own attendant cost and calibration problems. The use of magnetic wheels, if that is what is meant, would appear to present problems on account of surface contours, loss of engagement, and other factors. In particular, if there is a loss of engagement, it is likely that the gap created by the same will exceed the distance over which the magnets will operate to restore contact.